(CN4H7)H2C3N3S3: Trithiocyanurate Exhibiting Giant Optical Anisotropy via Weak Interaction-Mediated Functional Module Alignment

There is an immediate demand for identifying birefringent materials that possess substantial optical anisotropy to meet the requirements of photonic applications. Optimizing the orientation of functional modules to achieve enormous anisotropy is essential for enhancing the linear optical properties of birefringent materials. In this research, the arrangement of anionic groups was regulated by the hydrogen bonds formed between group (CN₄H₇)⁺ and (H₂C₃N₃S₃)^- groups, and coupled with the synergistic effect of the repulsive force between the same groups, this resulted in the design and synthesis of the first metal-free trithiocyanurate ultraviolet (UV) birefringent crystal (CN₄H₇)H₂C₃N₃S₃. It realized a robust trade-off between the short UV cutoff edge (336 nm) and large birefringence (0.319@546.1 nm). Remarkably, its UV cutoff edge is significantly blue-shifted, and its birefringence is much greater than that of all the documented (CN₄H₇)⁺-based compounds. According to theoretical calculations, the optical properties of (CN₄H₇)H₂C₃N₃S₃ are primarily influenced by (CN₄H₇)⁺ and (H₂C₃N₃S₃)^-. This study provides a viable approach for developing high-performance UV birefringent crystals in systems containing only π-conjugated groups.